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Vandervelde Earns Air Force Grant for Photodetection Research

Promising research in infrared photodetection has earned a Tufts University School of Engineering professor an early-career award from the Air Force Office of Scientific Research. The Air Force's Young Investigator Research Program (YIP) annually awards funding to U.S. scientists and engineers who show exceptional ability and promise for conducting basic research.

Assistant Professor Tom Vandervelde, the John A. and Dorothy M. Adams Faculty Development Professor in the Department of Electrical and Computer Engineering, received a YIP for his work on exploring and increasing the capabilities of photodetectors with applications for advances in biomedical diagnostics, health care, and sustainable energy.

Vandervelde will use the $450,000, three-year award to investigate the properties of nanostructure materials used to enhance the function of infrared photodetectors—devices that detect or measure electromagnetic wavelengths that are longer than wavelengths in the visible light spectrum.

Presently, infrared photodetectors are generally selectively tuned one particular wavelength. For example, a thermal imaging infrared camera photodetecting device measures the heat emitted by objects.

Tom Vandervelde is researching the design of new infrared photodetectors that could detect multiple wavelengths at the same time.

With a grant from the Air Force Young Investigators Program, Assistant Professor Tom Vandervelde is researching the design of new infrared photodetectors that could detect multiple wavelengths at the same time. Being able to interpret different wavelengths of light in the same photodetector could be useful when distinguishing between objects, such as being able to detect a person (above) obscured by foliage (below).
Photo credit: FLIR Systems Inc.

Heat is understood by the photodetector as a form of infrared electromagnetic radiation that can be seen by our eyes when translated in grayscale. Hotter objects emit more electromagnetic radiation and appear white; cooler objects appear dark.

"Traditionally, in the infrared, everything is effectively in black and white," says Vandervelde who says that infrared images are colored to make the distinctions more pronounced. "The color shows different emissivity—the ability of objects to emit electromagnetic radiation—it's not like how we think of color with different wavelengths; it's all one wavelength, and we're measuring the relative power intensity that's coming out."

To make photodetectors more useful, engineers like Vandervelde are working in an area known as “multimodal sensing”—the idea that one photodetecting device could detect multiple infrared wavelengths at the same time.

Presently, photodetectors require physically switching a filter to be able to understand different properties of an object, such as temperature or material composition. By using metmaterials, artificial electromagnetic composites, typically made of highly conducting metals, a filter could be created that could detect multiple wavelengths by a change in voltage.

"If I look at a white powder with a photodetector that senses temperature, I can say that this white powder is generally a certain temperature, but if use a photodetector with multiple wavelengths, I could actually identify what the material is, as well as its temperature," says Vandervelde, adding that in the infrared spectrum a white powder like anthrax could be distinguished from something like powdered sugar.

Being able to interpret different polarizations of light in the same photodetector is also useful when distinguishing between natural and man-made objects, Vandervelde says.

"Natural materials are not smooth, they're very rough. Manmade materials tend to be very smooth and hard, and they scatter light differently. If you can take an image and look at it from one polarization and look at it from another, manmade objects will stand out, whereas trees and natural materials won't," which could be used for search and rescue operations, looking for someone through a dense forest canopy from an airplane, for example, says Vandervelde.

In the biomedical field, a multimodal infrared photodetector could be used to detect patient hemoglobin and glucose levels at the same time, with the same device.

Though multimodal sensing devices with what are called "hyperspectral imaging" arrays do exist, says Vandervelde, they present challenges being highly complex and computationally intensive, wasting time and energy.

"If a photodetector camera has a 1000 by 1000 pixel array and each pixel interprets a thousand different wavelengths a second, that's a billion points of data," says Vandervelde. "It might take a week to figure out what you saw."

Without an active filtering mechanism designed to selectively interpret wavelengths, these devices detect all possible wavelengths and then an end user must sift through the data to find the relevant information.

"By increasing the functionality of the infrared photodetector, we can enhance the selectivity of the data actually recorded on the front end to reduce processing needs on the back end, thereby making real-time detailed information available,” whether at the patient bedside, in the military arena, or contaminated environmental sites, says Vandervelde.

The techniques used to create this tunable photodetector filter could also have energy applications in creating an anti-reflective coating for thermophotovoltaic devices and more traditional solar cells.

The AFOSR announced this year it will award more than $16.5 million in YIP grants to 43 scientists and engineers. According to AFOSR, the selection committee received 242 proposals in response to the office's broad agency announcement in major areas of interest to the Air Force. These areas of interest include: aerospace, chemical and material sciences; physics and electronics; and mathematics, information and life sciences.

The objective of this program is to foster creative basic research in science and engineering, enhance early career development of outstanding young investigators and increase opportunities for the young investigators to recognize the Air Force mission and the related challenges in science and engineering.

[story posted on November 10, 2010]